What is the status of divrei Torah written by someone who does not believe in the oral Torah? Does it retain any sanctity? Should it be burned or buried?
Nechama Leibowitz, in this letter, outlines her approach to this question. I think a basic level of intellectual honesty demands that if someone says a good pshat, that we use it. If they got it right, they got it right. To the sources regarding kisvei hakodesh written by an apikores, I think one must distinguish between the ideas and the physical books. The alternative is nonsensical. I'm supposed to ignore what I think is a correct idea simply because of who said it? Furthermore, on a practical level, I don't think the label of apikores can be so simply applied to all the "non-frum" sources. If someone did not grow up frum, its arguable their status is a tinok she'nishba, not an apikores. Additionally, it's debatable exactly what constitutes apikorsus. One man's Rebbe is another man's apikores. There's a lot of rishonim who did not conform to all 13 of Rambam's Ikkarim, yet they are accepted halachic authorities. The question of who and what is considered out of religious tradition is a much more complicated sugya than how it is presented in other answers to this question.
Rambam (Hilchos Mamrim 3:3): One who does not admit to oral Torah is an Apikores
Shulchan Aruch (334:21): If Kisvei ha'Kodesh were written by an Apikores, i.e. a devout idolater, or a Mumar to idolatry, we do not save it. Even on a weekday, we burn it with the Azkaros (mentions of Hash-m's name).
There is a story I verified about Rabbi Ben Zion Halberstam (Second Bobover Rabbi) in Bobov prior to WW2. Once a shipment of Seforim written by an Apikores arrived at the Yeshiva. He took them outside and burnt them all. At that time he composed the song B'ni Al Teileich B'Derech Itom.
The Gemara in Avoda Zara 17a relates how Rebbe Eliezer was mistaken for a Christian and he attributed this denigration to the fact that he once accepted a Torah idea from an heretic. The Gemara in Shabbos 75 likewise tells us that one who learns anything from a Magush (either a sorcerer or an heretic) is deserving of death.
Although we find that Rebbe Meir learned from Acher, it is made very clear that Rebbe Meir was different in that he was great enough, and knew enough to pick out right from wrong. For the rest of us, we have Rebbi Yochanan's statement that one should only learn from someone who is like an angel. It should also be pointed out that Rebbe Meir wasn't learning Chidushim from Acher but rather he was gleaning from him what was taught by the earlier Rabbeim.
This is also somewhat evident in the Sugya in Brachos 59, mentioned by Yehosef, where Rav Ketina laughed off the explanation of the necromancer although, as explained in the Gemara, his retort wasn't really true. However, the Gemara goes on to explain the approach of Rav Ketina, Rav Nassan, the Rabbanan, and Rav Acha bar Yaakov on that matter. None of them agree to the explanation given by the necromancer although Rav Ketina's rebuttal did not truly knock it down.